I like spirit shell in PvE because I also think the 3 dmg red is underwhelming and I’m not relying on it.
I see it more as a PvP talent because you can combine it with another PvP talent that greatly extends it’s duration and stacked with other damage reducing effects it adds up quick, so you can outlast the bursting.
What spirit shell allows you to do is stack shields in a high burst scenario allowing you to set up your damage and healing through atonement, where otherwise you’d have to possibly do something else, it saves you globals and allows you be more efficient and get on top of the incoming damage.
Rapture is also expensive to use to spam shields and not efficient because of it and you have to waste globalstalent (not on the shield itself) spirit shell makes better use of the talent and reduces globals and increases damage output downtime.
Also if you’re feared or cc’d or dealing with spread damage or a tough scenario with movement it allows you additional protection and buys you time.
You should cast two radiance casts if you want it to be an effective shield, this is the expensive part and it lasts a while and so you will also have time to pad the shield with other casts.
It flows better overall with a setup and makes a smoother rotation than having to throw in Rapture just awkwardly in there somewhere and it’s not as rewarding or intuitive as spirit shell.
It depends on how much damage you’re anticipating, you don’t have to cast radiance x2, but spirit shell should be used when targets have high health and you’re anticipating incoming damage, if you do it on targets with low hp it can feel awkward padding the shield instead of the health and then your target is still low on hp after the shield is gone and then you’re stuck having to recast heals on one or more players but other than that it’s a great talent.
It’s not complicated to use, read the talent to see what affects apply to spirit shell and read icy veins or whatever for the rotation, it’s not needed though, just practice with it and it’s intuitive enough, just make sure you understand how to play Disc effectively otherwise, like the basics and you should more than fine